Air separation method

ABSTRACT

An air separation method in which a liquid air stream, produced by vaporizing a pumped liquid oxygen stream, is introduced into a lower pressure column and optionally, a higher pressure column of an air separation unit. The liquid air stream is subcooled by extracting a main air feed to the higher pressure column from a main heat exchanger at a temperature warmer than the liquid air stream to increase argon recovery in an argon column connected to the lower pressure column. This temperature is selected such that the liquid air stream approaches an average temperature of the return streams being fed into the main heat exchanger from the higher and lower pressure columns at a range between about 0.2K and about 3K.

FIELD OF THE INVENTION

A method for separating air in which a pressurized oxygen product isproduced by vaporizing a pumped liquid oxygen stream against liquefyingan air stream in a main heat exchanger and an argon product is producedin an argon separation zone connected to a lower pressure column that isoperatively associated in a heat transfer relationship with a higherpressure column. More particularly, the present invention relates tosuch a method in which a main feed air stream to the higher pressurecolumn is withdrawn from the main heat exchanger at a temperature thatis warmer than the liquid air stream to subcool the liquid air stream,thereby to increase argon recovery.

BACKGROUND OF THE INVENTION

The separation of air into nitrogen, oxygen and argon fractions havebeen conducted in air separation units in which air is compressed,purified and cooled in a main heat exchanger to a temperature suitablefor its rectification. The air is introduced into a higher pressurecolumn of a double column arrangement also having a lower pressurecolumn in a heat transfer relationship with the higher pressure column.Nitrogen and oxygen products may be extracted from the higher and lowerpressure columns.

An argon-rich stream can be removed from the lower pressure column andintroduced into an argon column to produce argon-rich column overhead.The argon-rich column overhead is condensed, typically with the use ofall or part of a crude liquid oxygen stream, produced as a columnbottoms of the higher pressure column, to generate liquid reflux for theargon column. A portion of the argon-rich column overhead is taken as anargon product.

It is also known to produce a high pressure oxygen product in such anarrangement by pressurizing an oxygen-rich stream composed of a liquidoxygen column bottoms produced in the lower pressure column by pumpingthe stream and vaporizing it in the main heat exchanger againstliquefying an air stream that constitutes part of the air that has beencompressed to a high pressure. The resultant liquid air stream isexpanded and introduced into the lower pressure column or both thehigher and lower pressure columns.

An example of such a plant is disclosed in U.S. Pat. No. 6,293,126. Inthis patent, the main feed air stream is withdrawn from the main heatexchanger at a temperature warmer than that of the air stream that isfurther compressed and liquefied to produce the liquid air stream. In anattempt to simplify the construction of such a plant, the crude liquidoxygen stream is not subcooled either prior to its use in condensingargon reflux or its introduction into the lower pressure column. As aresult, there exists a greater vapor fraction of the crude liquid oxygenstream entering the lower pressure column after expansion than wouldhave otherwise occurred had the crude liquid oxygen stream beensubcooled. Thus, the liquid to vapor ratio at a point in the lowerpressure column above the point at which the argon-rich stream isextracted for further refinement in the argon column is less than wouldotherwise have been possible. Moreover, extracting a main air stream ata warmer temperature than the liquid air stream decreases thetemperature of the liquid air stream to an extent that it can approachthe temperature of the return streams used to cool the incoming air. Asa result, the compression requirements for the air stream that isfurther compressed and liquefied are usually greater than the flowand/or pressure that would otherwise have been required had the mainfeed air stream not been withdrawn at the warmer temperature. Thefurther subcooling of the liquid air stream tends to compensate for thereduced liquid to vapor ratio in the low pressure column. This resultsin more power being consumed in such a plant without any increase inargon recovery.

As will be discussed, the present invention provides a method forseparating air in which argon recovery is increased over that possiblein prior art air separation systems, such as discussed above, whileminimizing the amount of excess power that is necessarily used inincreasing the argon recovery.

SUMMARY OF THE INVENTION

The present invention provides a method of separating air.

In accordance with the method, a first compressed and purified airstream and a second compressed and purified air stream are produced. Thesecond compressed and purified air stream has a higher pressure than thefirst compressed and purified air stream. These streams are cooledwithin a main heat exchanger through indirect heat exchange with returnstreams that are produced in an air separation unit. The return streamsinclude at least part of a pumped liquid oxygen stream and as a resultof the indirect heat exchange, a main feed air stream and a liquid airare produced from the compressed and purified air.

The main feed air stream is introduced into a higher pressure column ofthe air separation unit and the liquid air stream is expanded and atleast part of the liquid air stream is introduced into a lower pressurecolumn of the air separation unit. An argon-rich stream from the lowerpressure column is introduced into an argon separation zone formed by atleast one column to produce an argon containing column overhead and anargon containing product stream composed of the argon containing columnoverhead. It is to be noted, that the term “argon separation zone” asused herein and in the claims includes a single argon column, oftenreferred to in the art as a crude argon column, as well as columns inseries that provide a sufficient number of separation stages that theargon product has very low levels of oxygen, typically less than about10 ppm.

A crude liquid oxygen stream composed of liquid column bottoms of thehigher pressure column and a nitrogen-rich liquid stream composed ofliquefied nitrogen column overhead of the higher pressure column aresubcooled. At least part of the crude liquid oxygen stream and at leastpart of the nitrogen-rich liquid stream are introduced into the lowerpressure column.

The main feed air stream is extracted from the main heat exchanger at atemperature warmer than that of the liquid air stream and introducedinto the higher pressure column at least at about such temperature.Preferably, the temperature of the main feed air stream is in a range ofbetween about 6K and about 25K warmer than the liquid air stream andmore preferably, in a range of between about 8K and about 15K warmerthan the liquid air stream.

The effect of this is to subcool the liquid air stream, therebyincreasing the liquid content thereof after having been expanded toimprove the liquid to vapor ratio in the lower pressure column andthereby to increase the argon recovery. It is to be noted that unlikethe prior art, there can be no simplification such as by not subcoolingthe crude liquid oxygen stream. If such stream were not subcooled, argonrecovery would suffer in that the liquid to vapor ratio above the pointof introduction of the liquid air stream or part thereof would be lessdue to evolution of vapor during the expansion of the same. Moreover,unlike the prior art, the temperature of the main feed air stream isselected such that the liquid air stream has an approach temperatureapproaching that of an average temperature of the return streams of noless than a range of between about 0.2K and about 3K, and preferablybetween 0.4K and 2K. The average temperature is a calculated temperatureat which a product of flow and enthalpy of the return streams at a coldend of the main heat exchanger is equal to the product of the flow andthe enthalpy of the return streams at their actual temperatures. As willbe discussed, it has been found by the inventors herein that if thistemperature is made any smaller, given the fact that a main heatexchanger is only of finite size, the compression requirement for thesecond compressed and purified air stream will increase with noappreciable increase in the argon recovery.

In order to overcome warm end and heat leakage, as well known in theart, refrigeration must be generated. There are a number of ways to dothis that are compatible with the present invention. For example, athird compressed and purified air stream can be produced. The thirdcompressed and purified air stream can be partially cooled within themain heat exchanger and introduced into a turboexpander to produce anexhaust stream for generation of the refrigeration. The exhaust streamcan then be introduced into the lower pressure column. A fourthcompressed and purified air stream can be produced by extracting thefourth compressed and purified air stream from an intermediate stage ofa compressor used in forming the second compressed and purified stream.The fourth compressed and purified stream is expanded within anotherturboexpander and combined with the first compressed and purified airstream within the main heat exchanger to increase liquid production.

As an alternative method of generating refrigeration, a nitrogen columnoverhead stream composed of the nitrogen column overhead can bepartially warmed within the main heat exchanger and then expanded withina turboexpander to produce an exhaust stream for the generation of therefrigeration. The exhaust stream can then be introduced into the mainheat exchanger and then fully warmed therein.

In any embodiment of the present invention, the liquid air stream can beintroduced into a liquid turbine to expand the liquid air stream to apressure suitable for its introduction into the intermediate location ofthe higher pressure column.

The crude liquid oxygen stream and the nitrogen-rich liquid stream canbe subcooled through indirect heat exchanger with return streams thatare formed from a nitrogen-rich vapor stream composed of column overheadof the lower pressure column and a waste vapor stream enriched innitrogen to a lesser extent than the nitrogen-rich vapor stream. Thenitrogen-rich vapor stream and the waste vapor stream can be introducedinto the main heat exchanger after having subcooled the crude liquidoxygen stream and the nitrogen-rich liquid stream.

A first part of the crude liquid oxygen stream can be expanded andintroduced into the lower pressure column and a second part of the crudeliquid oxygen stream can indirectly exchange heat with an argon columnoverhead stream composed of the argon column overhead. As a result, theargon column overhead stream can be condensed and the second part of thecrude liquid oxygen stream can be partially vaporized. Liquid and vaporfraction streams resulting from the partial vaporization of the crudeliquid oxygen stream can then be introduced into the lower pressurecolumn. Part of the argon column overhead stream after having beencondensed can form the argon product stream and a remaining part thereofafter condensation can be returned to the argon separation zone asreflux.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims distinctly pointing outthe subject matter that Applicants regard as their invention, it isbelieved that the invention will be better understood when taken inconnection with the accompanying drawings in which:

FIG. 1 is a schematic process flow diagram of an apparatus for carryingout a method in accordance with the present invention;

FIG. 2 is a graphical representation of the prior art heating andcooling curves in a main heat exchanger;

FIG. 3 is a graphical representation of the heating and cooling curveswithin a main heat exchanger that operates in connection with an airseparation method in accordance with the present invention;

FIG. 4 is a fragmentary, schematic view of an alternative embodiment ofFIG. 1 showing an alternative embodiment for a subcooling unitintegrated with the main heat exchanger;

FIG. 5 is a fragmentary, schematic view of an alternative embodiment ofFIG. 1 employing expansion of a nitrogen-rich stream to generaterefrigeration; and

FIG. 6 is a fragmentary, schematic view of an alternative embodiment ofFIG. 1 employing further expansion to increase the production of liquid.

In order to avoid repetition of the explanation of the accompanyingfigures, the same reference numerals are used and repeated in thefigures where the description of particular elements designated by thereference numerals are identical.

DETAILED DESCRIPTION

With reference to FIG. 1, an air separation plant 1 is illustrated thatis configured for carrying out a method in accordance with the presentinvention is illustrated.

An air stream 10 is compressed by means of a main air compressor 12. Theair pressure of the resultant compressed stream is set by the pressureof a higher pressure column 48 to be discussed hereinafter and pressuredrop. After cooling in an after cooler 14 to remove the heat ofcompression, the air stream 10 is purified within a purification unit 16to remove higher boiling impurities such as carbon dioxide and moisturethat could freeze as well as hydrocarbons that could collect to presenta safety hazard. Purification unit 16, as well known in the art, can bebeds of molecular sieve adsorbent operating out of phase in a knowntemperature swing adsorption cycle to purify air stream 10.

The compression and purification of the air stream 10 producescompressed and purified air stream 18 that is divided to produce a firstcompressed and purified air stream 20 that constitutes the largestportion resulting from such division. A part 22 of compressed andpurified air stream 18 is further compressed within a booster compressor24 to produce a second compressed and purified air stream 28. Part 22 ofcompressed and purified air stream 18 typically has a flow rate in arange of between about 24% and about 40% of compressed and purified airstream 18. The discharge pressure of the booster compressor 24 is set bythe pressure of a pumped liquid oxygen stream 122 also to be discussedhereinafter. When the pressure of second compressed and purified airstream 28 is below its critical pressure, the pressure is typically lessthan about 2.5 times the pressure of pumped liquid oxygen stream 122.The heat of compression of second compressed and purified air stream 28is preferably removed by after cooler 26.

As will be discussed, optionally, a further part 30 of compressed airstream 18 is compressed within a booster compressor 32 to produce athird compressed and purified air stream 36 for refrigeration purposes.The flow rate of the further part 30 of compressed and purified airstream is typically in a range of between about 5% and about 20% of thatof compressed and purified air stream 18. The heat of compression ispreferably removed from third compressed and purified air stream 36 byan after cooler 34. It is to be pointed out that main air compressor 12and booster compressor 24 are preferably multi-stage machines withinter-stage cooling. Booster compressor 32 is a single stage machinepowered by turbine 62. Compressors 12 and 24 are usually powered by anexternal source, usually an electric motor.

First compressed and purified air stream 20 and second compressed andpurified air stream 28 are cooled within the main heat exchanger 40 toproduce a main feed air stream 42 that is at or near its dew point and aliquid air stream 44. As will be discussed, first compressed andpurified air stream 20 and second compressed and purified air stream 28are cooled by indirect heat exchange with return streams, produced inthe air separation unit 46, that are enriched in oxygen and nitrogen. Itis to be pointed out that the present invention contemplates that secondcompressed and purified air stream 28 could be above the criticalpressure. In such case, the cooling of such stream would produce a densephase vapor in a process known as “pseudo liquefaction” in that noactual liquid phase would be produced. Therefore, the term“liquefaction” or the term “liquid” when used in connection with liquidair stream 44 herein and in the claims contemplates both a pseudoliquefaction that produces a dense phase vapor and an actualliquefaction that produces a liquid.

The main feed air stream 42 is introduced into a bottom region of ahigher pressure column 48 of air separation unit 46 that operates at ahigher pressure than a lower pressure column 50 of air separation unit.Air separation unit 46 also includes an argon column 52 that provides anargon separation zone for refinement of argon to produce an argoncontaining column overhead from which argon product is extracted. Argoncolumn 52 in a proper case could be replaced with a series of columns topresent a sufficient number of stages of separation to substantiallyseparate the oxygen as described above.

Although not illustrated, it is understood that higher pressure column48, lower pressure column 50 and argon column 52 contain mass transferelements to contact liquid and vapor phases of the mixtures to beseparated within such columns. These mass transfer elements can be knownstructured packing or sieve trays, dumped packing or combinationsthereof.

Liquid air stream 44 is introduced into a liquid expansion device 54 andis expanded to a pressure suitable for its introduction into anintermediate location of higher pressure column 48 above main feed airstream 42. Liquid expansion device 54, as illustrated, is preferably aliquid turbine in which the work of expansion can be recovered in anelectric generator, used to drive a compressor or dissipated as heatwith an oil brake. It is understood that liquid expansion device 54could be an expansion valve. After expansion, liquid air stream 44 isdivided into first subsidiary liquid stream 56 and a second subsidiaryliquid stream 58. The second subsidiary liquid stream 58 is introducedinto the higher pressure column 48. As such, the discharge pressure ofliquid expansion device 54 is set at a pressure of the higher pressurecolumn 48 plus pressure drop. The first subsidiary liquid stream 56 isreduced in pressure by an expansion valve 60 and then introduced intolower pressure column 50. As would occur to those skilled in the art,all of the liquid air stream 44 could be introduced into the lowerpressure column 50 and expanded to a suitable pressure for suchpurposes.

In order to refrigerate the process and thus, overcome warm end losses,third compressed and purified air stream 36 after removal of the heat ofcompression is partially cooled within the main heat exchanger 40. Bypartially cooled, what is meant is that the stream is cooled to atemperature that is between the warm and cold end temperatures of mainheat exchanger 40. The resultant third compressed air stream 36 afterhaving been partially cooled is then introduced into a turboexpander 62to produce an exhaust stream 64 that is introduced into the lowerpressure column 50. As is apparent from the illustration, the pressureof exhaust stream 64 is set at the pressure of the lower pressure column50.

The separation of the air within the higher pressure column 48 producesa nitrogen column overhead that is rich in nitrogen. Additionally, acrude liquid oxygen column bottoms is produced within higher pressurecolumn 48 that is enriched in oxygen. A nitrogen-rich vapor stream 66,composed of the nitrogen-rich column overhead, is introduced into acondenser reboiler 68 that is located within a bottom region of lowerpressure column to vaporize oxygen-rich liquid collecting as liquidcolumn bottoms within lower pressure column 50 against condensing thenitrogen-rich vapor stream 66 to produce the nitrogen-rich liquid stream70. Part 72 of nitrogen-rich liquid stream 70 is introduced back intothe top of higher pressure column 48 as reflux and a part 74 of thenitrogen-rich liquid stream 70 is subcooled along with crude liquidoxygen stream 76 composed of the crude liquid oxygen column bottoms ofhigher pressure column 48 in a subcooling unit 78.

Part 74 of nitrogen-rich liquid stream 70 is divided into first andsecond subsidiary nitrogen streams 80 and 82. Second subsidiary liquidnitrogen stream 82 can be taken as a product. First subsidiary liquidnitrogen stream 80 is reduced in pressure by an expansion valve 84 andthen introduced into the top of lower pressure column 50. As would occurto those skilled in the art, all of part 74 of nitrogen-rich liquidstream 70 could be introduced into lower pressure column 50.

An argon-rich stream 86 as a vapor is introduced into argon column 52.Argon-rich stream 86 will typically contain between about 5% and about20% argon. An argon-rich column overhead is extracted as an argon-richvapor stream 88 and condensed within a heat exchanger 90 located withina shell 92. The resultant argon-rich liquid stream 94, as a stream 96,is introduced back into argon column 52 as reflux and an argon productstream 98 can be extracted as an argon product. The resultant argon leanliquid stream 100 is returned to lower pressure column 50.

Depending upon the number of stages of argon column 52, argon-richcolumn overhead and therefore the argon product stream 98 can be a crudestream that requires further processing for purification purposes. Asknown in the art, such a crude stream can be further processed to removeresidual oxygen in a de-oxo unit and then in a nitrogen column to removeany residual nitrogen.

Crude liquid oxygen stream 76 after having been subcooled is thendivided and a first part 102 of such stream can be expanded within anexpansion valve 104 and directly introduced into lower pressure column50. A second part 106 can be expanded within an expansion valve 108 andthen introduced into the heat exchanger 92 in indirect heat exchangewith argon-rich vapor stream 88 to condense the same. The resultantvapor stream 110 can be introduced into the lower pressure column 50along with a liquid stream 112.

Crude liquid oxygen stream 76 and second part 74 of nitrogen-rich liquidstream 70 are subcooled within subcooling unit 78 through indirect heatexchange with nitrogen column overhead stream 114 and a waste stream 116having a lesser concentration of nitrogen than nitrogen column overheadstream 114. At the same time an oxygen-rich stream 118, extracted fromthe bottom of the lower pressure column 50, can be pumped by a pump 120to produce a pumped liquid oxygen stream 122. The pumped oxygen can alsobe above its critical pressure and therefore is a dense phase or “pseudoliquid.” The first part 124 thereof can be introduced into the main heatexchanger 40 for the liquefaction of second compressed air stream 28.Also introduced into main heat exchanger are other return streams suchas nitrogen column overhead stream 114 and waste stream 116. Thesereturn streams also serve to cool the incoming first compressed andpurified air stream 20 to produce the main feed air stream 42 and topartly cool the third compressed air stream 36. It is to be pointed outthat embodiments of the present invention are possible in which wastestream 116 is not removed. This results in the nitrogen column overheadstream 114 having a lower concentration of nitrogen and thus forming awaste stream. In the illustrated embodiment, however, column overheadstream 114, waste stream 116 and first part 124 of pumped liquid oxygenstream 122 consist of the return streams of the process.

Nitrogen column overhead stream 114 and the vaporized first part 124 ofthe pumped liquid oxygen stream form nitrogen and pressurized oxygenproducts. The second part 126 of pumped liquid oxygen stream 122 canoptionally be taken as a liquid product.

As indicated above, the first compressed air stream 20 is not fullycooled within main heat exchanger 40. Rather, it is withdrawn to producemain feed air stream 42 having a warmer temperature than the secondcompressed air stream 28 upon its liquefaction and discharge as liquidair stream 44 from main heat exchanger 40. As mentioned above, thiscauses the subcooling of liquid air stream 44. The temperature of mainfeed air stream 42 is preferably in a range of between about 6K andabout 25K warmer than liquid air stream 44. A more preferred range isbetween about 8K and about 15K.

With reference to FIG. 2, the temperature profile within the main heatexchanger 40 is shown in which the first compressed air stream 20 isfully cooled and is thus withdrawn after having fully traversed the mainheat exchanger 40. In this particular prior art operation, there existsa temperature difference in the cold end of main heat exchanger of about6.2K.

With reference to FIG. 3, the temperature profile within main heatexchanger 40 is shown in accordance with the present invention.Withdrawal of compressed and purified air stream 20 at the warmertemperature and therefore, production of main feed air stream 42 at thewarmer temperature results in a steeper cooling profile because all thatremains within the main heat exchanger 40 to be cooled is secondcompressed and purified air stream 28 which results in the production ofliquid air stream 24 at a subcooled temperature. As a result, lessvaporization occurs due to the expansion of liquid air stream 44 withinexpander 54 and the first subsidiary liquid stream 56 after passagethrough valve 60 and second subsidiary liquid stream 58 has a greaterliquid content upon its introduction into higher pressure column 48.Main feed air stream 42 is warmer entering the high pressure column.This results in greater liquid-vapor traffic and therefore an increasein the production of nitrogen-rich vapor in the top of higher pressurecolumn 48. The greater liquid content of first subsidiary air stream 56produces an increased liquid to vapor ratio below the point ofintroduction into the lower pressure column 50. Additionally, thegreater production of nitrogen-rich vapor at the top of higher pressurecolumn 48 results in more liquid being produced into lower pressurecolumn 50 as reflux by virtue of increased production of second part 74of liquid nitrogen-rich stream 70. In the present invention, since thecrude liquid oxygen stream 76 is also subcooled, a greater liquidfraction of this stream after expansion is also able to be introducedinto lower pressure column 50. The resultant overall greater liquid tovapor ratio within lower pressure column 50 results in more argon beingpresent within argon-rich stream 86 and therefore, a greater rate argonrecovery. It is to be noted, that the same also will increase the oxygenrecovery, albeit to a lesser extent. However, since, typically, theoxygen is being supplied to customers under supply contracts, the plantcan be operated to meet commercial needs by decreasing the degree ofmain air compression to also lower the overall power requirements of amethod conducted in accordance with the present invention while stilltaking advantage of the increased argon recovery possible in theinventive method disclosed herein.

However, as main feed air stream 20 becomes progressively warmer, thetemperature of the liquid air stream 42 becomes progressively lower. Inorder to prevent the heating and cooling curves within the main heatexchanger 40 from crossing, more air will have to be compressed withinbooster compressor 24 thereby increasing the power requirements of theplant. Increasing flow of 30 is another way of compensating for thesmaller temperature difference at the cold end of heat exchanger 40.This tends to increase total power and decrease argon recovery. It hasbeen found by the inventors herein that the withdrawal of the main feedair stream 20 from the main heat exchanger 40 at a specific, predefinedtemperature, allow the temperature of liquid air stream to be controlledso as to approach the temperatures of the return streams, namely,nitrogen column overhead stream 114, waste stream 116 and pumped liquidoxygen stream 124. Such control thereby allows for an increase in argonrecovery without unnecessary increases in the power requirements for thecompression of the air. In a typical plate-fin heat exchanger, the mainfeed air stream 42 should be withdrawn from the main heat exchanger 40at a temperature such that liquid air stream 44 has a temperature thatapproaches that of the average temperature of the return streams by noless than a range of between about 0.2K and 3K, and preferably between0.4K and 2K. Below this range in temperature, power requirements rapidlyincrease without any appreciable increase in argon recovery. Asmentioned above, this “average temperature” is calculated to be atemperature at which the flow times the enthalpy is equal to the flowtimes the enthalpy of such return streams at their actual temperature atthe cold end of the main heat exchanger 40. In the illustratedembodiment, the return streams at the cold end of main heat exchanger 40are first part 124 of pumped liquid oxygen stream 122, and nitrogencolumn overhead stream 114 and waste stream 116 at the warm end ofsubcooling unit 78. It is to be noted that if any additional streams arewithdrawn from the column system and then fed to main heat exchanger 40,then such streams would be counted in such calculation of the averagetemperature. As would be known, the control of such temperature of mainfeed air stream 44 is effectuated by design of the main heat exchanger40 and more specifically, the location of an outlet thereof to dischargemain feed air stream 42 therefrom.

With reference to FIG. 4, in an alternative embodiment of the airseparation plant shown in FIG. 2, main heat exchanger 40 and subcoolingunit 28 can be combined into a single unit 40′. The air separation plantillustrated in FIG. 4 otherwise functions in a manner set forth for theapparatus of FIG. 1.

With reference to FIG. 5, an alternative embodiment of the airseparation plant shown in FIG. 1 is illustrated. A nitrogen enrichedvapor stream 130 can be extracted from nitrogen-rich vapor stream 66 anda remaining portion 67 of nitrogen-rich vapor stream 66 can beintroduced into condenser reboiler 68. Nitrogen enriched vapor stream130 is introduced into main heat exchanger 40″ in which it is partiallywarmed and then introduced into a turboexpander 132 coupled to agenerator 134. The resultant cooled exhaust stream 136 is introducedinto the main heat exchanger 40″ that is provided with a passage tofully warm such stream and thereby refrigerate the process. Other thanthe alternative method of generating refrigeration, the plantillustrated in FIG. 5 is otherwise identical to that shown in FIG. 1.

With reference to FIG. 6, a yet further alternative embodiment of theair separation plant illustrated in FIG. 1 is shown. In such embodiment,a fourth compressed air stream 150 is taken from an intermediate stageof the booster compressor 24, preferably, the first or second stagethereof. The resulting fourth compressed air stream 150 is thencompressed within a compressor 152 to produce compressed air stream 154that, after removal of heat of compression within an after cooler 156,is introduced into a turbine 158 to produce an exhaust stream 160 thatis combined with first compressed air stream 20 at an intermediatelocation and temperature level of a main heat exchanger 40′″ having aninlet provided for such purpose. This results in a capability to producemore liquid than the plant shown in FIG. 1. Other than the modificationoutlined in this paragraph, the remainder of the plant would otherwisebe identical to the air separation plant 1 shown in FIG. 1.

The following are calculated examples of the operation of air separationplant 1, as illustrated in FIG. 1, that is conducted in accordance witha method of the present invention (Table 1) and a prior art method inwhich the main feed air stream 42 is withdrawn from the main heatexchanger 40 at the cold end temperature of the main heat exchanger 40(Table 2). In both examples, the plants are designed to produce aunitized gaseous oxygen flow of 1000 (first part 124 of pumped liquidoxygen stream 122 after vaporization in main heat exchanger 40) and aunitized liquid oxygen flow of 34 (second part 126 of pumped liquidoxygen stream 122).

TABLE 1 Pressure, Percent Stream Ref. No. Flow Temperature, K psiaComposition vapor  18 4948 282.0 88.0 air 100  20 2815 282.0 88.0 air100  28 (after cooling 1453 305.4 1100 air 100 in after cooler 26)  422815 108.9 84.0 air 100  44 1453 97.9 1099 air 0  58 436 96.2 83.7 air 0 56 (after valve 60) 1017 82.0 20.1 air 14.8  36 (after discharge 679144.9 136.8 air 100 from main heat exchanger 40)  64 679 89.2 20.2 air100  82 34.0 81.9 83.0 99.9998% 0 N₂ + Ar  98 36.1 89.1 17.8 99.9998% 0Ar 126 34.0 96.3 450 99.6% O₂ 0 124 (after 1000 291.0 446 99.6% O₂ 100vaporization within main heat exchanger 40) 116 (after being 815 291.017.2 98.6% N₂ 100 fully warmed within main heat exchanger 40) 114 (afterbeing 3029 291.0 16.9 99.9999% 100 fully warmed N₂ + Ar within main heatexchanger 40)

TABLE 2 Pressure, Percent Stream No. Flow Temperature, K psiaComposition vapor  18 4968 282.0 88.0 air 100  20 2863 282.0 88.0 air100  28 (after cooling 1426 305.4 1100 air 100 in after cooler 26)  422863 103.4 84.0 air 100  44 1426 103.4 1099 air 0  58 428 98.1 83.7 air3.9  56 (after valve 60) 998 82.1 20.1 air 20.2  36 (after discharge 679144.9 136.8 air 100 from main heat exchanger 40)  64 679 89.2 20.2 air100  82 34.0 82.0 83.0 99.9998% 0 N₂ + Ar  98 34.4 89.1 17.8 99.9998% 0Ar 126 34.0 96.3 450 99.6% O₂ 0 124 (after 1000 290.7 446 99.6% O₂ 100vaporization within main heat exchanger 40) 116 (after being 941 290.717.2 98.1% N₂ 100 fully warmed within main heat exchanger 40) 114 (afterbeing 2925 290.7 16.9 99.9999% 100 fully warmed N₂ + Ar within main heatexchanger 40)

By way of comparison, the argon recovery of the present invention, asshown in Table 1, is 78.1%. The argon recovery for a prior art method,represented in Table 2, is 74.1%. Likewise, the oxygen recovery fromTable 1 is 99.3%, the oxygen recovery in Table 2 is 98.9%. The lowerdegree of flash off of streams 56 and 58 as they enter the higher andlower pressure distillation columns 48 and 60, for the present inventionas shown in Table 1 (percent vapor), and the warmer temperature of mainfeed air stream 42, lead to the improved product stream recoveries. Thereduced flash off is a result of the lower temperature of liquid airstream 44 in the present invention. In Table 1, the flow of secondcompressed and purified air stream 28 is required to be 1.9% higher thanin Table 3. As a result, the power consumption for the present inventionis slightly higher than in the prior art.

While the invention has been described with reference to a preferredembodiment, as will occur to those skilled in the art, numerous changes,additions and omissions can be made without departing from the spiritand the scope of the present invention as recited in the appendedclaims.

1. A method of separating air comprising: producing a first compressedand purified air stream and a second compressed and purified air streamhaving a higher pressure than the first compressed and purified airstream; cooling the first compressed and purified air stream and thesecond compressed and purified air stream in a main heat exchanger,through indirect heat exchange with return streams produced in an airseparation unit that include at least part of a pumped liquid oxygenstream, thereby to produce a main feed air stream and a liquid airstream; introducing the main feed air stream into a higher pressurecolumn of the air separation unit, expanding the liquid air stream andintroducing at least part of the liquid air stream into a lower pressurecolumn of the air separation unit; introducing an argon-rich stream fromthe lower pressure column into an argon separation zone formed by-atleast one column to produce an argon containing column overhead and anargon containing product stream composed of the argon containing columnoverhead; subcooling a crude liquid oxygen stream composed of liquidcolumn bottoms of the higher pressure column and a nitrogen-rich liquidstream composed of liquefied nitrogen column overhead of the higherpressure column and introducing at least part of the crude liquid oxygenstream and at least part of the nitrogen-rich liquid stream into thelower pressure column; and the main feed air stream being extracted fromthe main heat exchanger at a temperature warmer than the liquid airstream and introduced into the higher pressure column at least at aboutsaid temperature, thereby subcooling the liquid air stream andincreasing the liquid content thereof after having been expanded toimprove the liquid to vapor ratio in the lower pressure column andthereby to increase the argon recovery, the temperature being selectedsuch that the liquid air stream has an approach temperature approachingthat of an average temperature of the return streams of no less than arange of between about 0.2K and about 3K, the average temperature beinga calculated temperature at which a product of flow and enthalpy of thereturn streams at a cold end of the main heat exchanger is equal to theproduct of the flow and the enthalpy of the return streams at theiractual temperatures.
 2. The method of claim 1, wherein the range isbetween about 0.4K and about 2K.
 3. The method of claim 1, wherein thetemperature of the main feed air stream is in a range of between about6K and about 25K warmer than the liquid air stream.
 4. The method ofclaim 1, wherein the temperature of the main feed air stream is in arange of between about 8K and about 15K warmer than the liquid airstream.
 5. The method of claim 4, wherein the range is between about0.4K and about 2K.
 6. The method of claim 1, wherein: the liquid airstream is expanded to a pressure suitable for its introduction into anintermediate location of the higher pressure column; the liquid airstream is divided into a first subsidiary liquid stream and a secondsubsidiary liquid stream; the first subsidiary liquid stream isintroduced into the higher pressure column; and the second subsidiaryliquid stream is expanded and introduced into the lower pressure columnabove a point of discharge of the argon-rich stream to the argon column.7. The method of claim 1, wherein: a third compressed and purified airstream is produced; the third compressed and purified air stream ispartially cooled within the main heat exchanger and introduced into aturboexpander to produce an exhaust stream for generation ofrefrigeration; and the exhaust stream is introduced into the lowerpressure column.
 8. The method of claim 5, wherein: a fourth compressedand purified air stream is produced by extracting the fourth compressedand purified air stream from an intermediate stage of a compressor usedin forming the second compressed and purified stream; and the fourthcompressed and purified stream is expanded within another turboexpanderand combined with the first compressed and purified air stream withinthe main heat exchanger to increase liquid production.
 9. The method ofclaim 1, wherein a nitrogen column overhead stream composed of thenitrogen column overhead is partially warmed within the main heatexchanger, expanded within a turboexpander to produce an exhaust streamfor generation of refrigeration and the exhaust stream is introducedinto the main heat exchanger and fully warmed therein.
 10. The method ofclaim 1 or claim 5 or claim 6 or claim 7 or claim 8 or claim 9, whereinthe liquid air stream is introduced into a liquid turbine to expand saidliquid air stream to the pressure suitable for its introduction into anintermediate location of the higher pressure column.
 11. The method ofclaim 1, wherein the crude liquid oxygen stream and the nitrogen-richliquid stream are subcooled through indirect heat exchange with thereturn streams that are formed from a nitrogen-rich vapor streamcomposed of column overhead of the lower pressure column and a wastevapor stream enriched in nitrogen to a lesser extent than thenitrogen-rich vapor stream, the nitrogen-rich vapor stream and the wastevapor stream being introduced into the main heat exchanger after havingsubcooled the crude liquid oxygen stream and the nitrogen-rich liquidstream.
 12. The method of claim 1, wherein: a first part of the crudeliquid oxygen stream is expanded and introduced into the lower pressurecolumn and a second part of the crude liquid oxygen stream indirectlyexchanges heat with an argon column overhead stream composed of theargon column overhead, thereby condensing the argon column overheadstream and partially vaporizing the second part of the crude liquidoxygen stream; liquid and vapor fraction streams resulting from partialvaporization of the crude liquid oxygen stream are introduced into thelower pressure column; and part of the argon column overhead streamafter having been condensed forms the argon product stream and aremaining part thereof after condensation is returned to the argonseparation zone as reflux.